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On 2016-03-29 20:13, N Drummond wrote:
> The Wild T-4 instrument can read directly to .1 second (that's 1 in  12.96 
million divisions).  Key to this capabilty was a very large glass horizontal 
circle, an impersonal eyepiece micrometer to read the horizontal glass 
circle, and of course the incredibly precise optical and mechanical 
fabrication capability of the Wild company.

The impersonal micrometer measures the transit of a star through the
field of view during longitude determinations. It has nothing to do with
reading the circles.

In the old days a star moved across a set of stationary crosswires. Each
time it coincided with a wire the observer pressed a switch. Due to
variations in reaction time and visual perception, each observer had a
systematic bias, or "personal equation".

The impersonal micrometer replaces the fixed wires with a movable wire.
Rotation of the micrometer screw to keep the wire on the star also
rotates a disk with electrical contacts in its edge. A chronograph
records the contact closures and the 1-second pulses from the clock as
pen traces on a moving piece of paper.


> - the large horizontal circle (240mm, or 24cm, or 9.5 inches)  provided a 
basic instrument capable of 2-second readings  by scaling the same design 
capability already in use with the Wild T-2 and T-3.    For comparison, the 
Wild T-2 can be read directly to .4 seconds, with a horizontal circle of 
8.5cm; the T-4 design uses a glass circle 3X larger with the abilty for 
measuring about 3.3X greater divisions compared to the T-2 (.1 sec   vs .4 
sec)

The T2 circle has 10 minute graduations. Its micrometer is directly
readable to 1 second, i.e., that's the interval of the graduations.
(There was also a T2 version which measured angles in gons.)

A direct reading to .4 second isn't going to happen since that's not a
neat subdivision of a degree.


> - the optical micrometer at the eyepice further divided the 2-second reading 
into 20 divisions-  so 2-seconds, divided into 1/20- became .1 second direct 
reading.
> (think of using a microscope with 20-divisions/ intervals across the 
field-of-view  to read the horizontal circle markings)
> The key technology that provided this capability is due to the use of 
photo-lithographic methods to mark the horizontal circle, not mechanical 
engraving- The T-4, with a 240mm (24cm/ 10inch) wide circle, had a lot more 
room for photo-lithographically etching the needed markings to allow 2-second 
graduations on the circle.

No, that's completely wrong. The T4 horizontal circle is graduated every
2 *minutes* (a logical extension of the T3 circle, which is about half
the diameter and graduated every 4 minutes). In the reading microscope,
opposite sides of the circle are visible simultaneously, one above the
other in a split image. The lower half is inverted and moves right when
the upper half moves left. Thus the graduations in the split image
coincide every minute and the circle is directly readable to 1 minute,
sans micrometer.

The micrometer has a 1 minute range. An optical mechanism shifts the
halves of the split image in opposite directions when the micrometer
knob is rotated. When the graduations coincide, degrees and minutes are
read from the split image. Seconds are read from a dial (graduated every
tenth), a portion of which is visible in the lower part of the
microscope field.

A T2 or T3 user will be immediately comfortable with this system. In
fact, the micrometers on all three theodolites divide their range into
600 parts.

The University of New South Wales instrument collection includes a T4.
In the photo, the impersonal micrometer drive knobs (one at each end of
a common shaft) are visible at the left end of the horizontal axis. They
could be mistaken for focus knobs. Below them, the horizontal circle
reading microscope protrudes from the upright. It can be folded down
when not needed. On the upright, at the base of the microscope, is the
silver knob of the micrometer.

The T2 and T2 have a knob to select either the horizontal or vertical
circle. Both circles share the same microscope and micrometer. But these
are separate on the T4. The vertical circle eyepiece is the little black
thing at the extreme right of the picture. The micrometer knob is out of
view on the other side.

http://www.sage.unsw.edu.au/currentstudents/ug/projects/f_pall/html/

US Army manual TM 5-442, Precise Astronomic Surveys, has much
information on determination of latitude, longitude, and azimuth with
the Wild T4.

https://archive.org/details/armytechnicalman003486mbp

The NOAA archive has some photos of the T4 in action. In the picture of
the guy recording observations, I think the device with a paper strip
coming out of a slot is a chronograph. The old fashioned chronographs
made a helical trace on a rotating drum.

http://www.photolib.noaa.gov/brs/geind22.htm
http://www.photolib.noaa.gov/htmls/geod1128.htm
http://www.photolib.noaa.gov/htmls/geod1129.htm
http://www.photolib.noaa.gov/htmls/theb1537.htm
http://www.photolib.noaa.gov/htmls/theb1538.htm

   

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